fix Claude pane hang: cap tail, parse off-isolate, coalesce notifies

The native Claude pane (T-137) picks the newest transcript JSONL by
mtime — which on a live session is this multi-MB active file. On attach
it read and parsed the whole file synchronously on the UI isolate and
fired notifyListeners per item, freezing the app.

Three fixes: cap the initial read to a recent tail (256KB, injectable
for tests); run JSON parsing in a background isolate via Isolate.run;
coalesce the controller's notifications with a zero-duration Timer so a
burst collapses into one rebuild (a microtask-scheduled notify wouldn't
— stream events deliver one per microtask and the notify interleaves).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -97,6 +97,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- Claude pane no longer freezes the app on open — the transcript reader
+  caps its initial read to the recent tail, parses off the UI isolate,
+  and coalesces view notifications into one rebuild per burst (T-137).
 - Daemon-not-connected on startup — panels and the Claude pane raced
   the socket loopback. Requests now wait briefly for an in-flight
   connection, the server isn't restarted for the same workspace, and